1. Field of the Invention
The present invention relates to a dielectric filter and a dielectric duplexer for use in a high-frequency circuit, and to a communication apparatus provided with the dielectric filter or the dielectric duplexer.
2. Description of the Related Art
Dielectric filters provided with a plurality of resonance lines in a dielectric block are disclosed in the following three publications.
Japanese Unexamined Patent Application Publication No. 9-64616 discloses a dielectric filter in which a recess (slot) inside which a conducting film is formed is provided on the open-end face of a dielectric block in order to capacitively couple resonance lines to each other.
Japanese Unexamined Patent Application Publication No. 5-335808 discloses a dielectric filter in which a groove inside which a conducting film is formed is provided at an end face of a dielectric block so that a capacitance is generated between areas near the open ends of resonance lines and the groove.
Japanese Unexamined Patent Application Publication No. 10-256807 discloses a dielectric filter in which each resonant through hole has a stepped form. The central axis of a large-diameter hole is largely deviated from the central axis of a small-diameter hole to form a flexed resonant through hole.
Generating attenuation peaks by coupling through holes to each other and setting the pitch between the through holes as required allows the frequencies of attenuation peaks to be controlled to desired frequencies.
FIGS. 9A to 9D show a structure example of a known dielectric duplexer; FIG. 9A is a top view of the dielectric duplexer, FIG. 9B is a front view of the dielectric duplexer, FIG. 9C is a bottom view of the dielectric duplexer, and FIG. 9D is a right-side view of the dielectric duplexer. A substantially rectangular-parallelepiped dielectric block 1 of the dielectric duplexer has a plurality of resonant through holes 2a to 2c, 3, 4a to 4d, and 5, each having a conductor formed inside. Each ground hole 6 is provided between the resonant through holes 2a and 3, between the resonant through holes 2c and 4a, and between the resonant through holes 4d and 5 in order to block the coupling therebetween. A conducting film is formed inside the overall ground hole 6, and the opposing ends of the conducting film are connected to an outer conductor 10.
The dielectric block 1 also has excitation through holes 7, 8, and 9, each having a conductor formed inside. The outer conductor 10 is formed on the outer surfaces of the dielectric block 1. One end of each of the excitation through holes 7, 8, and 9 is connected to the outer conductor 10 on one end face of the dielectric block 1. A transmission terminal 17, an antenna terminal 18, and a reception terminal 19 are formed on the other ends of the excitation through holes 7, 8, and 9, respectively, and extend toward a surface to be mounted on a board. The resonant through holes 2 to 5 are stepped holes, each having a larger inside diameter at its open end (the opposing side of the front side in FIG. 9B) and having a smaller inside diameter at its short-circuited end (the front side in FIG. 9B). In the structure example in FIGS. 9A to 9D, a resonator formed by the through hole 4a is capacitively coupled (C coupling) to a resonator formed by the through hole 4b, a resonator formed by the through hole 4b is inductively coupled (L coupling) to a resonator formed by the through hole 4c, and a resonator formed by the through hole 4c is capacitively coupled (C coupling) to a resonator formed by the through hole 4d. 
FIG. 10 is a graph showing a transmission characteristic between the antenna terminal 18 and the reception terminal 19 of the dielectric duplexer in FIG. 9A to 9D. An attenuation peak Pab occurs by coupling the resonator formed by the through hole 4a to the resonator formed by the through hole 4b. An attenuation peak Pbc occurs by coupling the resonator formed by the through hole 4b to the resonator formed by the through hole 4c. An attenuation peak Pcd occurs by coupling the resonator formed by the through hole 4c to the resonator formed by the through hole 4d. The pitch between the through holes 4c and 4d is set to be smaller than the pitch between the through holes 4a and 4b so that the attenuation peak Pcd occurs near the passband.
In any of the dielectric filters disclosed in the above publications, the resonant through holes are arranged at pitches set in accordance with the characteristics in order to couple the resonators to each other with predetermined degrees of capacitive or inductive coupling. In other words, since the frequencies of the attenuation peaks are varied in accordance with the pitches between the resonant through holes, the frequency positions of the attenuation peaks with respect the passband are controlled in accordance with the pitches between the resonant through holes. As a result, in many cases where three or more resonant through holes are provided, the pitches between the resonant through holes are varied, as described above.
However, since the variation in size of the molded dielectric block is associated with the variation in electrical characteristics of the dielectric filter, areas in which the pitches between the resonant through holes are small are largely affected by the variation in size of the molded dielectric block, thus leading the increase in a defective factor in the manufacturing. Furthermore, the current density flowing through the resonance through holes is increased in the areas in which the pitches between the resonant through holes are small and, therefore, unloaded Q (Q0) of the resonators is decreased. This constitutes a factor in preventing the characteristic improvement due to the provision of the attenuation peaks.